A gaseous reactor effluent containing acrylic acid, acetic acid, water and various organic impurities is obtained by the oxidation of propylene or acrolein over an oxidation catalyst. This gas stream is then condensed or absorbed in water to obtain an aqueous solution of acrylic acid, acetic acid and water.
The concentration of acrylic acid in this solution varies. In a fixed-bed reactor, where diluents such as steam may be used, a final aqueous solution of about 10 wt.% acrylic acid is obtained. Where diluents are not used, the resulting aqueous solution can contain about 30-70 wt.% acrylic acid.
Because of the relative volatilities of acrylic acid and water, simple distillation is extremely difficult. Therefore, many different methods of recovery of acrylic acid have been proposed. For example, U.S. Pat. No. 3,816,524 discloses the use of various solvents to extract acrylic acid. The extraction is then followed by stripping the solvent from the acrylic acid in a distillation column.
Another method of recovery is azeotropic distillation. U.S. Pat. No. 3,830,707 discloses a process wherein a specific entrainer of isooctane or nitroethane is used to perform the distillation.
Both of these methods have various drawbacks. Distillation even under vacuum involves subjecting the acrylic acid to temperatures that usually cause some polymerization of the acid. Solvent extraction requires many columns, a high solvent to feed ratio, and suffers high solvent losses.
A variation on solvent extraction is by the addition of certain salts such as sodium chloride to accelerate phase separation after extraction, as shown in U.S. Pat. No. 3,846,488.
Other specific salts have been used for various purposes. U.S. Pat. No. 3,663,375 describes a process for salting out organics such as methacrylic acid using electrolytes such as sodium sulfate. These salts have not been effective for acrylic acid however, due to the difference in structure between the acids.
Finally, U.S. Pat. No. 2,922,815 discloses that drying agents such as calcium chloride can be used to obtain a 50-60% acrylic acid solution; dry metal salts such as nickel chloride or nickel bromide can obtain concentrations of up to 80% acrylic acid; and that it is not possible to obtain higher concentrations in this manner.
The present invention, through the use of specific salts, allows concentrations of between 90-100% acrylic acid to be obtained by phase separation at very desirable temperatures.